Prestressed cathode structure for electron discharge devices

ABSTRACT

A planar-type cathode for electron discharge devices which is in relatively close-spaced relation with another electrode and which is provided with means for preventing bowing toward the electrode under thermal stresses.

United States atent $1101] Feb. 8, 1972 [54] PRESTRESSED CATHODE STRUCTURE [56] References Cited FOR ELECTRON DISCHARGE DEVICES UNITED STATES PATENTS [72] Slmu, East Nmwalk 2,135,941 11/1938 Hirmann ..313/337 x 7 Assigneez The Machle Laboratories lncorporawd 2,310,81 l 2/1943 Schantl 6! al. ....313/270 X springdale Conn 2,138,918 12/1938 Haller ..313/337 [22] Filed: Apr. 2, 1970 Primary Examiner-David Schonberg Assistant ExaminerToby H. Kusmer [21] App! 25130 AttorneyHar0ld A. Murphy 52] us. c1 ..313/270, 313/337 [571 'W [51] ..H0lj 1/94, H0 lj 19/4 A planar-type cathode for electron discharge devices which is [58] Field of Search ..313/270, 337, 37, 310 in relatively close-spaced relation with another electrode and 3 which is provided with meansfor preventing bowing toward the electrode under thermal stresses.

8 Claims, 4 Drawing Figures 2 /4334: J/ is 32 41 4 2;; 34 4, 7/ 6 46 3s Q 3e 5O 7/ 6 4 56 so 3 I 180 54 /42 a Ii 1 IQ Q1 40 fl PATENTEBFEB a m 3.641 .387

SHEET 2 [IF 2 Fig 4.

a0 a2 14 as 7 2 8 6 W e e F [gt 3 I/VVE/VTOR HERBERT L. SHOLL AGE/V PRESTRESSED CATHODE STRUCTURE FOR ELECTRON DISCHARGE DEVICES BACKGROUND OF THE INVENTION In the manufacture of electron discharge devices such as power tubes or cathode-ray tube guns, for example, which employ substantially planar cathode electrodes, it is often necessary to position another electrode such as a grid in extremely close proximity to the emitting surface of the cathode. However, it has been found that when the cathode is subjected to thermal stresses, resultant expansion occurs and often leads to the condition known as oil canning," that is, bowing outwardly of the central portion of the cathode. This produces undesired engagement of the cathode with the grid, destroying or at least severely interfering with efficient operation of the tube.

In some tubes the grid is located as near as 0.005 inches to the cathode. Consequently, thermal expansion of the cathode in the direction of the grid easily produces shorting. This is particularly true in tubes which are designed to have fast warmup cathodes and to achieve this the cathode is made extremely thin, as thin as 0.002 inches, for example. When a resistance-type heater near the cathode is activated, the cathode becomes radiatively heated and, being thin, quickly conducts heat to an electron-emissive coating on the cathode surface. However, in doing so, thermal expansion causes the cathode to bow, sometimes quite suddenly, as described above.

One attempt to overcome this problem is described in US. Pat. No. 3,056,060 which is assigned to the same assignee as the present invention and which discloses a cathode which is initially slightly concave or bowed in a direction away from the grid electrode. However, this is not always possible since it is desirable to maintain the planar cathode and grid electrodes in parallel predetermined spaced relation, and in such a concave cathode structure the spacing thus is nonuniform. Even with concave cathodes, upon thermal expansion the cathode often will oil can toward the grid.

In some instances the cathode heater may be placed so close to the rear surface of the cathode as to prevent oil canning of the cathode in a direction away from the grid, and may even assist in causing such motion toward the grid.

SUMMARY OFTl-IE INVENTION The above and other objections to and disadvantages of the prior art are overcome in the present invention by the provision' of a planar cathode which has means for preventing bowing of the cathode when subjected to thennal stresses, which means may take the form of an extruded truss in the center of the cathode. The truss may be a dimple which, when being made as by punch and anvil, introduces stresses in the material. When the cathode is subsequently heated, relaxation of these stresses occurs before thermal expansion. Thus, the cathode will not bow. The truss also provides an additional mechanical resistance to bowing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial sectional view through a power tube which employs a cathode embodying one form of the present invention;

' FIG. 2 is a top plan view of the cathode only of the tube illustrated in FIG. 1;

FIG. 3 is an axial section through the cathode of an electron gun of a type employed in cathode ray tubes; and

FIG. 4 is an enlarged vertical sectional view through a portion of a cathode embodying a modified form of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, and particularly to FIG. 1, there is shown a power tube of a generally conventional type which includes an evacuated envelope comprised of three spaced wall sections 12, 14 and 16 of ceramic, glass or other dielectric material between each pair of which are sealed annular metal terminal rings 18 and 20, preferably of kovar. An anode block 22 is sealed within an opening in one end of the envelope by a kovar sleeve 24, and a copper pinchoff tubulation 26 at the opposite end of the envelope is similarly sealed by a kovar ring 28 to ceramic ring 16, thus completing the envelope.

For convenience, the terminals 18 and 20 each initially comprise two separate rings l8a-l8b and 20a-20b respectively which are sealed to respective ceramic rings and later, during final assembly, to each other.

Terminal ring 18b has an inner cylindrical longitudinally extending grid-supporting member 30 which has a grid 32 mounted transversely on its upper end. Grid 32 spans a gridsupporting ring-34 by which it is afi'rxed to the end of member 30. An annular grid retainer 36 encircles the edges of ring 34 and grid 32 and assists in retaining the grid properly in place, and further has a number of spaced projections orfingers 38 extending downwardly therefrom into physical contact with the inner edge of terminal ring 18a for providing a suitable rf current path.

Terminal ring 20b has an inner cylindrical axially extending cathode-supporting portion 40 to which is welded or otherwise secured one end of a hollow cylinder 42 of foil, on the upper end of which is a cathode 44. The cathode 44 comprises a disc of suitable material such as nickel, for example, having a downwardly extending peripheral flange or rim 46 by which the disc is affixed to the supporting foil 42.

The surface of the cathode which is directed toward the grid 32 is coated with a layer 48 of electron-emitting material such as barium or a mixture of barium and strontium, for example. The layer 48 is preferably about 10 mils in thickness and lies very close to the grid, actually only a few millimeters away from it in accordance with conventional tube parameters so as to provide the desired grid-cathode capacitance.

Near the opposite side of the cathode disc is a coiled heater 50 of the flat pancake type having one end formed as a leg 52 which is connected by metal connector 54 to the supporting ring 40. Thus, the terminal ring 20b, supporting ring 40, connector 54, and leg 52 form one side of an electrical circuit to the heater 50. The other end of the heater coil forms a leg 56 at or near the center of the coil, which leg 56 extends downwardly into one end of a length of metal tubing 58 and into a metal sleeve 60 located within tubing 58. The tubing 58 and sleeve 60 are mechanically crimped around leg 56 to firmly hold the leg immovably in place. In this way the heater may be adjusted in proper spaced relation to the cathode disc. The sleeve 60 is affixed to one end of a connector 62, the other end of which is fixed to the ring 28. Thus, the electrical circuit is completed between the heater 50 and the ring 28 through leg 56, sleeve 60, tubing 58, and connector 62.

The upper end of tubing 58 may carry one or a number of metal discs or rings 64, for providing a heat bafile to deflect heat toward the cathode. One of the discs may be a getter, if desired. Such a conventional heater structure is fully described in US. Pat. No. 3,056,060 issued to the same as signee as the present invention. According y, more detail thereof is not provided herein.

The cathode 44, in many tubes of the character described, is made extremely thin, and may be as thin as about 0.002 inches in thickness to achieve fast warmup. It has been found that in such cases the cathode very often oil cans toward the grid; that is, when thermally expanded, the central portion of the grid pops or physically suddenly moves toward the grid. Such oil canning often causes grid to cathode shorts, high-cutoff readings, increased grid-cathode capacitance, etc. Attempts to overcome this problem by making a concave cathode, as shown, have failed to solve the problem.

Therefore, in accordance with this invention, the cathode is provided in its center with a truss point 66 which provides a stress within the material of the cathode disc and also mechanically strengthens the disc. The truss point 66 is a depression or similar deformation at the center of the disc,

with the depression being in the direction away from the grid. One suitable truss point was a round depression about 0.11 inches in diameter and about 0.0055 inches deep in the center of a cathode disc with a 0.0010 inches concavity. This truss point occupied about 8.6 percent of the total emitter area, and was made by a punch and anvil process.

It was found that upon being heated, the stress introduced into the material of the cathode disc will first relax, and any thermal expansion will occur thereafter and will be in the direction of the heater. Of course, the particular mechanical shape of the dimpled cathode disc will also aid in resisting movement of the disc.

Although the foregoing description is directed primarily to a cathode structure in a power tube, it will be readily apparent that the invention may be utilized as well in other types of tubes. For example, cathode ray tubes employ a planar cathode which is closely spaced to another electrode and problems thus are similar to the problems in power tubes.

FIG. 2 illustrates a cathode structure 66 and grid 68 of the type used in the electron gun of a cathode ray tube. Cathode structure 66 includes a hollow metal cathode cylinder 70 closed at one end by portion 72 which is coated with a layer 74 of electron-emitting material. The material of the cathode cylinder 70 and end portion 72 is thin so as to provide fast warmup capabilities, being about 0.003 inches thick, for example. The emitting layer 74 may be about 0.002 inches to 0.004 inches thick and may comprise barium oxide or strontium oxide, or a mixture thereof.

Within the cathode cylinder 70 is a heater element 76 which is adapted to be connected to a suitable source of electrical potential for the purpose of providing a heat source for heating the end portion 72 and the emitter layer 74 thereon whereupon the emitter will emit a copious supply of electrons.

Surrounding the cathode structure 66 is the grid 68 which is in the form of a cup having an axially extending cylindrical wall 78, and an end wall or effective grid portion 80 which closely overlies the cathode, being only about 0.005 inches, for example from the emitter 74. Grid portion 80 is provided with a central aperture 82, about 0.020 inches in diameter for example, through which the beam of electrons from layer 74 passes. The cathode 66 and grid 68 are positioned and held in predetermined spaced relationship by a dielectric ring 84 as shown, and other spacing and retaining elements (not shown) may also be employed.

It is apparent that when the cathode end wall portion 72 is quickly heated, thermal expansion can cause it to become bowed, often in the direction of the closely adjacent grid wall 80. In accordance with the present invention, this problem is overcome by providing the wall portion 72 with a truss point 86 similar to truss point 66 in the structure of FIG. 1. In a cathode of this type for use in an electron gun it is believed preferable that the diameter of the dimple or truss point 86 not be substantially greater than the thickness of the emitting layer 74. This is illustrated in FIG. 4 by a truss point 86 which is of extremely small diameter but relatively deep. Layer 74 will extend into the depression so that there will be no discontinuity in the emitting surface. However, the entire emitting surface may be a continuous unbroken plane as shown in FIG. 4, or may be depressed slightly to follow the contour of the cathode.

When heat from heater element 76 is absorbed by cathode wall 72 the stress, which was introduced into the wall by fabrication of the dimple 86, will first relax, after which some thermal expansion of the wall may take place. In this event, thermal expansion will appear as sight movement of the wall in the direction of the heater because of the mechanics of the truss point.

From the foregoing it will be apparent that a novel and improved cathode structure for electron discharge devices has been provided in accordance with the objectives of this inven-- tion. It is to be understood, however, that various modifications and changes may be made by those skilled in the art without departing from the spirit of the invention as ex ressed in the accompanying claims. Therefore, all matter set orth is means is a heater element positioned adjacent the side of the effective cathode portion opposite said surface.

3. A cathode as set forth in claim 2 wherein said truss point is a depression in the effective portion of the cathode, which depression extends into said surface and is defined by a projection extending from the opposite side thereof, in a direction toward the heater element.

4; A cathode as set forth in claim 3 wherein said first surface has thereon a layer of electron-emitting material.

5. A cathode as set forth in claim 4 wherein said layer is of a predetermined thickness, and the truss point is of a diameter which is not greater than the thickness of the layer.

6. An electron discharge device comprising a vacuumized envelope, anode and cathode electrodes in spaced relation within the envelope, a third electrode between said cathode and anode electrodes, the cathode electrode having a substantially planar effective portion with a surface positioned in close-spaced relation to the third electrode, and means for heating said effective portion of the cathode electrode, said portion having a truss point therein and being normally stressed whereby relaxation in such stress will occur when the portion is heated by said means.

7. An electron discharge device as set forth in claim 6 wherein said heating means is a heater element positioned adjacent the side of the effective cathode portion opposite said surface, and the truss point is a depression in said efi'ective portion of the cathode, which depression extends into said surface and is defined by a projection extending from the opposite side thereof toward the heater element.

8. An electron discharge device as set forth in claim 7 wherein said first surface has thereon a layer of electronemitting material of predetermined thickness, and the truss point is of a diameter which is not greater than the thickness of the layer. 

1. A cathode for an electron discharge device, comprising a heating means, and a cathode electrode to be heated by said heating means and having a substantially planar effective portion with a surface adapted to be close-spaced to another electrode, said portion having a truss point therein and being normally stressed whereby relaxation in such stress will occur when the portion is heated by said means.
 2. A cathode as set forth in claim 1 wherein the heating means is a heater element positioned adjacent the side of the effective cathode portion opposite said surface.
 3. A cathode as set forth in claim 2 wherein said truss point is a depression in the effective portion of the cathode, which depression extends into said surface and is defined by a projection extending from the opposite side thereof, in a direction toward the heater element.
 4. A cathode as set forth in claim 3 wherein said first surface has thereon a layer of electron-emitting material.
 5. A cathode as set forth in claim 4 wherein said layer is of a predetermined thickness, and the truss point is of a diameter which is not greater than the thickness of the layer.
 6. An electron discharge device comprising a vacuumized envelope, anode and cathode electrodes in spaced relation within the envelope, a third electrode between said cathode and anode electrodes, the cathode electrode having a substantially planar effective portion with a surface positioned in close-spaced relation to the third electrode, and means for heating said effective portion of the cathode electrode, said portion having a truss point therein and being normally stressed whereby relaxation in such stress will occur when the portion is heated by said means.
 7. An electron discharge device as set forth in claim 6 wherein said heating means is a heater element positioned adjacent the side of the effective cathode portion opposite said surface, and the truss point is a depression in said effective portion of the cathode, which depression extends into said surface and is defined by a projection extending from the opposite side thereof toward the heater element.
 8. An electron discharge device as set forth in claim 7 wherein said first surface has thereon a layer of electron-emitting material of predetermined thickness, and the truss point is of a diameter which is not greater than the thickness of the layer. 